Re: querry related to STL map..
Thomas Maeder wrote:
"ravips" <ravips123@gmail.com> writes:
I have a map<char *,int> it does not work consistently.
It does. You just don't like the semantics it has.
I have used similar sort of implementation as below:
map<char *,int> m1;
I can think of very, very few cases where this map might make
sense. Do you really want to map addresses to int's?
More likely, you want to map C-style strings to ints (judging
from the code which follows). In that case:
1. you need to provide a comparison function or functional
object, with its type as the third instantiation argument of
map, and
2. you need to do something to ensure that no one modifies the
C style string once it is in the map---using char const* is
the easiest solution.
In practice, I generally find it a lot easier to just map
std::string to whatever. That takes care of all of the above
issues, plus any memory management issues which crop up using
the map.
map<char *,int>::iterator i;
m1["January"] = 31;
Side note: The type of "January" is 'array of 8 char const'.
The implicit conversion of a string literal to 'pointer to
char' (non-const!) that this code relies on is deprecated.
Better make the key type of your map char const *.
Agreed, but I think you'd agree that making it std::string would
be even better.
m1["February"] = 28;
m1["March"] = 31;
m1["April"] = 30;
m1["May"] = 31;
m1["June"] = 30;
m1["July"] = 31;
This is similar to the map that i have used. Now accessing
this yields different results as below:
Option1:
char *p = "May";
int it = m1[p]; // Works properly returns 31.
Whether it returns 0 or 31 is not specified. The implementation
may merge identical string literals or not. And since the index
is the address of the data, and not the data itself, you get
different results depending on whether the string literals have
been merged or not.
Option2:
char *p = new char[3];
strcpy(p, "May");
This copies 4 char objects to an array that can only hold 3
char objects. Your program has undefined behavior.
int it = m1[p]; // returns 0 .. instead of 31 ....
That's one instance of undefined behavior.
Actually, even correcting the undefined behavior (which isn't
actually likely to cause a problem in the case of "May", because
most memory allocators cannot allocated an odd number of bytes
exactly, and round up), the standard requires 0 here. There's
no way that the string literal used to initialize the entry
could possibly have the same address as dynamically allocated
memory.
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